The present invention relates to a method of manufacturing a composition for magnets made of resinous material (referred to as a bonded magnet hereinbelow).
Conventionally, the composition material for bonded magnets, i.e. resin bonded magnets, is generally prepared by mixing ferrite powder into matrices, for example, of rubber, plastics or the like, and in many cases, such known composition contains 80 to 90% by weight of ferrite powder.
For kneading the composition material as described above, devices such as kneaders, rolls, etc. are available. However, since each of such devices is arranged to provide the kneading effect by subjecting the composition material to a compression or shearing during the kneading, a very high abrasion tends to occur at metallic surfaces of the device which are brought into contact with said composition material. Thus, each time the kneading is repeated, the compressing and shearing performances of the device are deteriorated, with consequent deviations of kneading conditions in a continuous operation. This results in such disadvantages that the bonded magnets prepared thereby have unstable magnetic characteristics largely deviating from one kneading lot to another. Furthermore, owing to the abrasion as described above, countermeasures such as replacement of tanks, blades, rolls, etc. for the kneading devices, or application of surface treatments and the like become necessary. This gives rise to a considerably large economical loss.
Moreover, since the ferrite powder is present in a high ratio in the composition, the dispersion of the ferrite powder and matrices in the composition tends to be uneven. This requires strict maintenance of specific conditions during the kneading, requiring a skilled worker to attend the kneading operation at all times, with consequent difficulties for automation of the work. Furthermore the matrices of resin material are liable to be deteriorated and lose their strength due to a long period of time required for uniform kneading.
Moreover, owing to the fact that a small amount of additive contained in the composition material is apt to scatter or adhere to a mixing vessel during the mixing thereof with the ferrite powder and matrices, mixing through strict measurement is difficult to achieve. This results in significantly large deviations in the composition.